This invention relates to balanced mixer circuits in general and more particularly to a broadband balanced mixer for microwave applications.
An extensive amount of work has been done in the field of monolithic microwave integrated circuit (MMIC) components in order to create a fully monolithic front end, as for example, for a RF receiver. This group of components has been extensively investigated in the prior art but, in any event, an area that receives little attention relates to monolithic mixers.
A primary reason is that most system performance requirements dictate balanced or double balanced mixers which use 180.degree. hybrids. Conventional passive hybrids, using transmission lines, are physically too large at microwave frequencies to be implemented in practical monolithic applications requiring low cost and high volume. Essentially, as is known, the microwave mixer circuit converts an RF signal to an intermediate frequency (IF) signal. This intermediate frequency (IF) signal is usually chosen to fall in the range of 20-100 MHZ. The frequency is low enough to build high quality IF amplifiers relatively inexpensive and yet high enough to avoid the flicker noise that is inversely proportional to frequency (1/f noise).
The IF frequencies are obtained by injecting, in addition to the RF signal, a local oscillator (LO) or pump frequency w.sub.p. The difference or IF frequency is a result of circuit non-linearities. The non-linearity effect in implementing mixer circuits is well known. In any event, double balanced mixers are also well known in the prior art and are capable of isolating both the RF signal and local oscillator voltages from the output. Hence, the term double balanced is used. In such circuits, such as the ring circuit, all diodes are pointed in the same direction, while in a star circuit two diodes point towards the central node and two away from it. In any event, the operation of such devices is well known in the art.
For an example of certain present day microwave mixers, reference is made to a text entitled "Microwave Semiconductor Circuit Design" by W. Alan Davis published by Van Nostrand Reinhold Company (1984). Pages 253-263 entitled "Mixers" describe various prior art devices.
Pursuant to these investigations, there have been many articles written concerning microwave mixing devices. See for example an article entitled "Broadband Monolithic Single and Double Ring Active/Passive Mixers" by A. M. Pavio et al. and published in the IEEE 1988 Microwave and Millimeter-Wave Monolithic Circuits Symposium. This article describes a balun topology that is implemented using monolithic technology. The balun circuit uses common gate and common source circuit techniques employing an ideal 180.degree. phase shifter for signals present between the upper and lower halves of the balun circuit. Essentially, the article describes dual mode characteristics of distributed broadband baluns in diode mixer topologies whereby one can achieve a relatively compact monolithic circuit which is designed to operate over a frequency range several octaves wide with performance comparable with conventional passive diode mixers.
Reference is also made to an article entitled "A GaAs Double-Balanced Dual-Gate FET Mixer IC for UHF Receiver Front-End Appiications" by K. Kanazawa et al. published in the IEEE Transactions on Microwave Theory and Techniques, Volume MTT-33, No. 12, December, 1985, pp. 1550-1553. In this article there is described a double balanced dual gate FET mixer for application in the front end of UHF receivers. The mixer utilizes GaAs double balanced dual gate FET's which are arranged on an integrated circuit and fabricated on the basis of a model of the dual gate FET mixer. In any event, the circuit diagram is shown in FIG. 1 of the article.
As one can ascertain from the above prior art, the structure of mixer devices for microwave application are extremely complicated and, as indicated, are very difficult to fabricate in practical monolithic IC applications requiring low cost and high volume. Furthermore, many of the systems utilize passive hybrid configurations which passive devices employ transmission lines and are, as indicated, too large at microwave frequencies to be implemented in practical embodiments.
It is therefore an object of the present invention to provide an improved broadband monolithic balanced mixer which can be integrated for microwave applications.
It is a further object of the present invention to provide a broadband monolithic balanced mixer which is small in size, which provides high isolation and where the output of the mixer can be directly coupled to additional circuitry.